1. Field of the Invention
The present invention described herein relates generally to an intake manifold flange that is used for joining an intake manifold and a cylinder head of an internal combustion engine to enable a predetermined sealing movement to accommodate heat and vibration stresses.
2. Description of Related Art
For example, in an internal combustion engine installed on a motorcycle, the air that has been filtered by an air cleaner passes through an intake manifold and is supplied to the cylinder head and interior of the cylinder. The joining portion of the intake manifold and cylinder head is usually jointed by using an intake manifold flange (referred to herein below simply as “flange”). Since the flange is also required to function as an insulator gasket, each gap between the flange and other members is sealed using an elastic seal gasket.
FIG. 3 is a cross-sectional view illustrating the configuration of the seal gasket portion of a conventional flange. As shown in FIG. 3, a flange 2 formed of metal and a seal gasket 40 formed of rubber join and seal an intake manifold 3 and a cylinder head 4.
Thus, an intake port 4a is provided in the cylinder head 4, and a distal end of a cylindrical intake manifold 3 is joined to the intake port 4a. In this case, the inner diameter of the intake port 4a of the cylinder head 4 is equal to the inner diameter of the intake manifold 3 so as not to inhibit the flow of an air-fuel mixture inducted into the cylinder head 4.
The flange 2 is a round disk or elliptical disk shaped member, having in the center thereof an opening 2a for inserting the distal end of the intake manifold 3. The inner diameter of the opening 2a is somewhat larger than the outer diameter of the intake manifold 3 to enable the aforementioned insertion. The flange 2 has two bolt insertion holes 2b in the circumferential edge portion thereof, and the flange 2 and the cylinder head 4 are connected to each other by screwing the distal ends of separately prepared bolts 5 into a wall surface of the cylinder head 4 via the insertion holes 2b. 
The seal gasket 40 is a ring-shaped member having in the center thereof an opening 40a for inserting the distal end of the intake manifold 3. The inner diameter of the opening 40a is equal to or somewhat less than the outer diameter of the intake manifold 3. Since the seal gasket 40 is formed from an elastic rubber material, the inner surface of the opening 40a and the outer surface of the intake manifold 3 are sealed by the elastic deformation of the seal gasket 40.
In particular, in the conventional configuration, the seal gasket 40 has a prominently triangular cross-sectional shape (wedge shape) with its widest thickness at the intake manifold 4 side. In order to mount the seal gasket 40 of such prominently triangular cross-sectional shape, a taper portion 2c expanding toward the cylinder head side is formed at the inner circumference of the flange 2 on the cylinder head 4 side. As a result, when the flange 2 is tightened to the cylinder head 4 by the bolts 5, the seal gasket 40 sandwiched therebetween is pressed against the taper portion 2c of the flange 2 and the seal gasket 40 is brought into intimate contact with the surface of the cylinder head 4 and the outer circumferential surface of the intake manifold 3.
Such conventional technique of using the flange and seal gasket is disclosed, for example, in Japanese Patent Application Publication No. H11-132121 and Japanese Patent Application Publication No. 2006-342773.
Generally, when an internal combustion engine operates and is warmed up, the cylinders and cylinder head 4 undergo thermal expansion and contraction. In the running state, in particular in a V-type engine, the expansion and contraction cycles cause changes in the joining distance of the intake manifold positioned in the place sandwiched between two cylinders. As a result, the bolt-tightened cylinder head 4 and the flange 2 slip on the manifold located inside the seal gasket, and move as shown by an arrow in FIG. 3. A similar phenomenon occurs not only when an internal combustion engine is operated, but also when oscillations of the cylinder head 4 are transmitted to the intake manifold 3 during operation of a vehicle or a machine in which an internal combustion engine is installed.
However, in the conventional configuration shown in FIG. 3, since the seal gasket 40 closing the gap between the flange 2 and the intake manifold 3 is strongly tightened by the bolts 5 fixing the flange 2 to the cylinder head 4, whereby the sealing objective is attained, the intake manifold 3 cannot follow the movement of the flange 2 or cylinder head 4.
Therefore, the inner surface of the seal gasket 40 and the outer circumferential surface of the intake manifold 3 rub against each other, or the seal gasket 40 is deformed. As a result, deterioration or wear of the seal gasket 40 occurs, and eventually a gap is formed between the intake manifold 3 and the seal gasket 40, seal performance deteriorates, and secondary air is inducted into the intake manifold 3.
Where the secondary air is inducted from the gap between the intake manifold 3 and the cylinder head 4, the balance of the intake mixture becomes unstable, and when the air-fuel mixture becomes lean and easily flammable, various inconveniences—for example, backfire—increase and operational instability or stalling, can occur. Further, when heat or gasoline is discharged from inside the manifold through the gap in the joining portion, the seal gasket 40 is scorched or becomes swollen.
This phenomenon clearly appears in a V-type engine of a motorcycle such as described in Japanese Patent Application Publication No. 2008-180129. FIG. 4 is a schematic diagram of an engine described in Japanese Patent Application Publication No. 2008-180129. This engine is provided with a first cylinder 23a and a second cylinder 23b that are arranged in a V-shaped configuration with respect to a crankshaft 22 inside a crankcase 21. A cylinder body 24, a cylinder head 4, and a combustion chamber 30 are provided in each of the cylinders 23a, 23b, and the combustion chamber 30 is connected to an intake manifold 3 at the entrance to an intake port 4a. 
In such a V-type two-cylinder engine, when the first and second cylinders 23a, 23b extend or shrink in the axial direction thereof (A direction in FIG. 4) due to thermal expansion, the distance between the two cylinders changes and a force is applied to the intake manifold 3 in the B direction shown in FIG. 4. As a result, in the configuration in which strong fixing is performed by the flange 2 and the seal gasket 40, as in the related art illustrated in FIG. 3, the action of this thermal expansion cannot be followed and the above-described problems occur.
By contrast, with the technique described in Japanese Patent Application Publication No. H11-132121 and Japanese Patent Application Publication No. 2006-342773, a flange is fixed to the distal end of the intake manifold by welding, integral molding, or casting, and the leak of the secondary air from the gap between the flange and the intake manifold is prevented. However, when the existing intake manifold that has not been integrally molded with a flange is connected to a cylinder head, the manifold of the above-described type cannot be used due to the structure thereof.
In particular, in existing engines for motorcycles, an intake manifold and a flange are often produced separately for improved attachment maintainability, and the structures described in Japanese Patent Application Publication No. H11-132121 and Japanese Patent Application Publication No. 2006-342773 cannot be used. When the related art illustrated in FIG. 3 is used in such a motorcycle, the leak from the connection portion of the intake manifold and cylinder head can not be reliably prevented over a long period.